This invention relates to a method of adjusting the resistance of a chip resistor of the type for mounting to a chip by a laser-trimming process.
FIG. 5 shows a prior art chip resistor of this kind, characterized as comprising a ceramic substrate 1, a resistor 2, a pair of electrodes 3 and a glass coating layer 5. Such a chip resistor may be produced first by using a silver paste or the like to form the electrodes 3 on the ceramic substrate 1 and then by using a resistor paste or the like to form the resistor 2 so as to connect the pair of electrodes 3. After the glass coating layer 5 is formed over the resistor 2, a laser beam is used for a laser-trimming process to form a cross-sectionally wedge-shaped groove 4 in the resistor 2 through the glass coating layer 5, reaching the top surface of the ceramic substrate 1, so as to adjust the resistance value of the resistor 2. A protective layer may be formed thereafter over the glass coating layer 5.
If the groove 4 is thus formed, the resistance of the resistor 2 increases because the current which can flow therethrough is necessarily reduced. Thus, when the resistor 2 is initially formed, it is formed such that its resistance will be smaller than the desired resistance of the chip resistor to be obtained and the laser-trimming is effected so as to appropriately increase the resistance of the resistor 2 to the required target value.
Although FIG. 5 shows an example with an L-shaped groove 4, the groove 4 may be cut in other forms. FIG. 6A shows an example of a C-shaped groove 4, FIG. 6B shows another example with a J-shaped groove 4 and FIGS. 6C and 6D show still other examples with I-shaped grooves 4.
FIG. 7 shows equipotential lines 6 in the resistor 2 of FIG. 5 when a potential difference is applied across its electrodes 3. If the applied potential difference is 200V, a potential difference of about 150V will result between positions A and B on the resistor 2 shown in FIG. 7 which are on the opposite sides of the groove 4. If the gap across the groove 4 is 50 xcexcm, the field intensity between these two points A and B is about 3000V/mm, and it is approximately equal to the voltage at which an atmospheric discharge will start. Thus, a discharge may well take place across the groove 4, depending on the condition of the glass coating layer 5 over the resistor 2. In summary, a leak current is likely to flow through such a prior art chip resistor when it is subjected to a high potential difference. In other words, prior art chip resistors are not satisfactorily resistant against high potential differences.
FIGS. 8A, 8B, 8C and 8D show equipotential lines inside the chip resistors of FIGS. 6A, 6B, 6C and 6D. When a same potential difference is applied across the pair of electrodes 3, the field intensity is about 635V/mm between positions A and B of FIG. 8A, about 3730V/mm between positions A and B of FIG. 8B, about 1570V/mm and 1590V/mm respectively between positions A and B and positions Axe2x80x2 and Bxe2x80x2 of FIG. 8C, and about 2090V/mm and 1800V/mm respectively between positions A and B and between Axe2x80x2 and Bxe2x80x2 of FIG. 8D. In summary, the possibility of atmospheric discharge is equally high in the case of a J-shaped groove as in the case of an L-shaped groove. The field intensity is also fairly high in the cases of I-shaped grooves.
If the groove is C-shaped, by contrast, the field intensity across the groove is relatively small. As a practical problem, however, a C-shaped groove (as shown in FIGS. 6A and 8A) is difficult to make with a satisfactorily high precision. When such a groove is cut by a laser-trimming method, the trimming is started at an edge point of the glass coating layer (not shown in FIG. 6A or 8A) to first produce an L-shaped groove, and the resistance is adjusted then to a specified target value. The direction of movement of the laser is then changed by 90 degrees, while the laser light continues to be emitted, and the groove is formed to reach the same edge of the glass coating to complete a C-shape. When it is desired to change the direction of movement of the laser after the groove has been formed in an L-shape, however, the galvanometer which is disposed inside the laser-assisted manufacturing apparatus tends to fluctuate due to inertia. As a result, the L-shaped groove tends to become longer than desired, or it is difficult to form the groove in the desired shape with a high degree of precision. This means that the resultant resistance of the chip resistor tends to be higher than the target value.
It is therefore an object of this invention in view of the above to provide a method of carrying out laser-trimming in making a chip resistor having highly accurate resistance value.
A chip resistor embodying this invention, with which the above and other objects can be accomplished, may be characterized as having grooves formed in a different pattern having a longer branch and a shorter branch, the longer branch extending between a selected point on a side edge of the resistor which is longitudinally elongated between a pair of electrodes and an end point which is longitudinally displaced from the selected point towards one of the electrodes and a shorter branch extending between another point on the side edge of the resistor and an intermediate point on the longer branch other than its end point. The longer branch, for example, may be in an L-shape with a part perpendicular to the longitudinal direction of the resistor and another part which is substantially in the longitudinal direction. To form the grooves in such a pattern, the longer branch is formed first by laser-trimming from its selected point to the end point. The laser is then switched off and is moved to the intermediate branching point. The shorter branch is then formed by switching on the laser and moving it from the branching point to the other end thereof on the same side edge of the resistor.
With a chip resistor thus formed, the area of the resistor surrounded by the longer and shorter branches of the grooves is not affected by the potential difference applied across the electrodes. As a result, the distance between the high-potential area and the low-potential area of the resistor is effectively increased and the likelihood of a leak current is reduced. With a method of laser-trimming according to this invention, the dimensions of the grooves can be controlled more dependably and accurately.